Please read to the end. We’ve done a lot so far and I’m looking for NEW ideas.

Okay. So I’ve got a 2001 Outback that is giving my mechanic and I a run for our money. 

So far we have replaced:

- Both head gaskets

- Thermostat (twice!)

- water pump & timing belt

- radiator 

Here’s what happens:

Driving in the city, heat on - no temp change

Driving on the highway, heat on - overheats when idling, not while driving 

If I turn the temp to cold, the gauge goes back to normal, instantly. 

Today my mechanic bypassed the heater core because we suspected it was clogged. The car still overheated, after being driven at highway speed for 30 minutes, I turned it off, the coolant was at a normal level, the upper rad hose was Boiling and the lower one was warm, but firm. It took less than ten minutes to get back down to normal temp, and did not overheat the whole drive back. When I got home the coolant was at the same level but the upper hose and lower hose were the same temp.

anything you can think of that hasn’t been tried yet? 

 

 

Are the Fans coming on as they should?

Have  you checked the temp of the heater hoses?

When you turn the temp to cold, you say the gauge goes back to "Normal".  What happens to the temp of the upper and lower hoses before and after, when you do this.  It makes little sense as  you are controlling the airbox flaps, not the actual coolant flow.

If you  have an OBDII reader, you may want to track the coolant temp there as well.

1. The most important question is are you 100% absolutely certain that the cooling issues never changed *at all* after those repairs?  The issues you have now are identical to the issues you had before any work was done?

2. Did you ever own the car when it wasn’t doing this, or did you buy it this way?

New radiator or used?

Fans?

Check the AC condenser in front of the radiator for clogged fins or egregiously bent fins. It may not be getting any airflow.

3. What brand headgaskets were used?

4. Were the heads resurfaced and bolts properly torqued?

preferably, answer all of the questions above, they work better together not individually.

 

First thing I would do, is hook up to the OBD II port and watch temperatures. The factory gauge will read "normal" from about 180*F to about 215*, and will be pegged in the red by about 225, so it only takes a few degrees to make the gauge move, if it's right at the edge. Also, only takes a small error in resistance for a 210* engine to read as a 230* engine, one is a problem, the other isn't, the ECU gets it's own temperature signal. The lack of a change in coolant level makes me question whether it's overheating at all.

I would also use an infrared heat gun ($20 or less on Amazon), and start checking various coolant pipes, and spots on the engine block/heads directly.

I want to clarify, it does not overheat on the highway? Or ONLY while idling?

I ASSume mention of the timing belt means this is a 4-cylinder, yes?

Is the temperature effected by any of the other HVAC controls? How about now that the core is bypassed. On the H4 cars, the temperature control is a cable to a blend door that directs air through or around the heater core. It's a very simple circuit. But it is crucial to making the thermostat work.

The difference in temperatures in the radiator hoses means the coolant isn't circulating. Either there's a water bubble getting to the water pump and causing it to cavitate, or there's a blockage (could just be the thermostat doing it's job if it's not actually overheating).

I echo the questions of heater hose temperature, and head gasket brand/application. Also brand of water pump, water pump gasket, and thermostat. Was any coolant stop leak ever used? Was the coolant ever changed? How many miles on the car?

More than likely we're looking at headgasket failure - but we'd be better off with specific details and larger context. 

coupla comments;

were the thermostats used OEM style with the larger wax capsule? it matters

have u  tried test runs with thermostat removed or 'drilled-out' ?